1. Field of the Invention
The invention relates to inkjet printing apparatuses and fabrication methods for color filters using the same, and in particular to an atmospheric plasma inkjet printing apparatuses and fabrication methods for color filters using the same.
2. Description of the Related Art
Conventional front-end processes for fabricating thin film transistor liquid crystal display (TFT-LCD) devices includes fabrication of a color filter (CF) substrate, a thin film transistor substrate, a driver IC, a backlight module and liquid crystal materials. Among the production costs for conventional TFT-LCD devices, material costs account for about 60% to 70%. Particularly, costs of the color filter can reach almost 1/4 of the total material cost of the TFT-LCD devices. As applications for TFT-LCD devices become wider and wider, demand for color filters also increases.
In order to solve defect problems during fabrication of pixellization structures such as color filters, pixel electrodes, black matrices, photo spacers etc., a method for fabricating the structure, for example, well known in fabrication of color filters using an inkjet printing system has been explained. Recently a new color filter fabrication method by inkjet printing has been developed, requiring a precisely controlled platform dispensing inkjet droplets on predetermined patterned pixel regions. The color droplets on the patterned pixel regions must not overflow to adjacent pixel regions to prevent color mixing. Moreover, the pattern and resolution of the color filters depend greatly on the nozzle interval of an inkjet printhead, resulting in increased fabrication complexity.
Conventional method for fabricating color filters includes forming color ink droplets directly on a color filter substrate confined within concavities defined by a black matrix (BM). Each color filter element is dispersed with different color ink droplets consisting of a red (R), a green (G) and a blue (B) color dots as a unit pixel. Compared with other conventional semiconductor fabrication methods, formation of a color filer using inkjet printing can dramatically reduce both equipment and production costs. Conventional inkjet printing, however, requires precise position alignment to ensure the ink droplet are dispersed at the predetermined position. Moreover, since it is difficult to uniformly diffuse the ink droplet within the concavities, defects such as white omission can be formed in the concavities.
To solve the above mentioned problems, conventional methods for fabricating a color substrate using inkjet printing provides an ink absorption layer in the concavities of the color filter substrate. The ink droplets are coated with a special arrangement which includes determining dimensions and locations of the ink droplets. Then, the ink droplets are diffused to desirable regions due to the high diffusion capability of the ink absorption layer. The conventional method can further incorporate an optical calibration alignment to provide more accurate and precise alignment.
However, formation of the ink absorption layer can increase production cost and fabrication process complexity. After the ink droplets are absorbed and diffused, white omission and color mixing can still exit between adjacent concavities, thereby severely deteriorating color filter quality. Furthermore, optical calibration alignment is achieved by decoding analogue signals, i.e., the printing location is determined by distributions of light passing through the concavities due to slot effect. Distributions of light can be affected by the relative location between the light source and charge coupled device (CCD), resulting in peak-to-valley shifts and affecting precision of determining the printing location.
U.S. Pat. Nos. 5,984,470, 6,145,981, 6,207,984, and 5,847,720, the entireties of which are hereby incorporated by reference, disclose inkjet printing apparatuses and fabrication methods for color filters. A precise platform is required to control ink droplets printed in a predetermined pattern. Note that the ink droplet must not diffuse to adjacent color elements during fabrication to prevent color mixing. The patterns and resolutions of the color filter must rely on the distance between printhead nozzles. Particularly, the printhead nozzles can be easily clogged due to dried ink.
FIG. 1 is a schematic view of a conventional fabrication method for a color filter using inkjet printing. A conventional inkjet printing apparatus 20 injects ink droplet 30 on a substrate 10 confined within concavities defined by a black matrix (BM) 12. If the substrate 10 is untreated by a surface treatment, the ink droplet 30 prints globular ink droplets due to surface tension and the hydrophobic surface. The globular ink droplets are more difficult to control. Thus, the dimensions and location of the globular ink droplets are unpredictable after being dried. For example, a convex ink droplet 30a can be formed at a pixel region due to strong surface tension, as shown in FIG. 2A. Alternatively, a concave ink droplet 30b can be formed at a pixel region due to weak surface tension, as shown in FIG. 2B. Furthermore, conventional untreated substrates can result in misalignment of the ink droplet 30c over the pixel regions resulting in color mixing, as shown in FIG. 2C.
Accordingly, a market demand for a surface treatment method to change surface characteristics such that surface tension between the ink droplet and the substrate can be reduced. The contact angle between the ink droplet and the substrate is less than 10° to form a flatten ink droplet in the pixel region, as shown in FIG. 2D. Furthermore, the ink droplet can further be self-aligned due to surface tension differences between adjacent heterogeneous regions.